The note that a wine glass makes when you hit it is its resonant frequency – the frequency at which the glass vibrates most efficiently. If you can match this tone you can smash the glass. The note will be around one octave above middle C, depending on the size of the glass. The real trick is singing loudly enough. Even with expensive,glasses, which vibrate with a clear tone that doesn’t damp down quickly, the amplitude required is at the limit for the human voice. An untrained singer can do it relatively easily with good amplification but achieving the same thing without a speaker has only been done once on TV, by singer and voice coach Jaime Vendera for the Discovery Channel’s Mythbusters show.